1. Field of the Invention
This invention relates to apparatus for testing power circuits and, more particularly, to devices for testing power circuits including ground fault and/or arc fault circuit interrupters. The invention also relates to a method for testing a power circuit including a ground fault circuit interrupter.
2. Background Information
The common type of circuit breaker used for residential, commercial, and light industrial applications has an electromechanical thermal-magnetic trip device to provide an instantaneous trip in response to a short circuit and a delayed trip in response to persistent overcurrent conditions. Some of these circuit breakers include ground fault protection, which trips the ground fault circuit interrupter (GFCI) in response to a line-to-ground fault, and in some cases, a neutral-to-ground fault. Ground fault protection is provided by an electronic circuit which is set to trip at about 4 to 6 mA of ground fault current for people protection, and at about 30 mA for equipment (or earth leakage) protection. It is known to incorporate a test circuit in the circuit breaker, which tests at least portions of the electronic ground fault trip circuit.
It is also known to test for improper wiring connections (i.e., line and ground being reversed; line and neutral being reversed; ground being open; excessive ground resistance; neutral being open; line being open). Test circuits for this purpose are commercially available.
Although test circuits and devices for testing for improper wiring connections are known, there is room for improvement in the variety of tests that are performed.
Recently, there has been rising interest in also protecting power distribution circuits, and particularly the branch circuits for homes, commercial and light industrial applications, from arc faults. Arc faults are intermittent, high impedance faults, which can be caused for instance by worn or damaged insulation, loose connections, broken conductors and the like. Arc faults can occur in the permanent wiring, at receptacles, or more likely, in the wiring of loads or extension cords plugged into a receptacle. Because of the intermittent and high impedance nature of arc faults, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnetic trip device which provides the short circuit and overcurrent protection.
Various types of arc fault detectors have been developed and/or proposed. Generally, the detectors are of two types. One type responds to the random high frequency noise content of the current waveform generated by an arc. This high frequency noise tends to be attenuated, especially by the presence of filters on some loads, which can be connected to the branch circuit. The other basic type of arc fault detector responds to the step increase in current occurring as the arc is repetitively and randomly struck. Examples of arc fault detectors of the latter type are disclosed in U.S. Pat. Nos. 5,224,006; and 5,691,869.
U.S. Pat. No. 5,459,630 discloses several forms of built-in test circuits for arc fault detectors. In one embodiment, in which the arc fault detector utilizes a coil to sense current, the test circuit adds a capacitor which forms with the impedance of the coil an oscillator generating a waveform with an amplitude which simulates the rapid rise of a step change in current produced by an arc. In another embodiment, the user must repetitively close a switch, which connects a resistor between the line conductor and neutral, to again generate large amplitude pulses.
While the built-in arc fault and ground fault testers test the response of the electronic circuits to simulated conditions, they do not necessarily indicate whether the device will adequately respond in a real installation. One difficulty is that the circuit breaker containing the detectors is located at a load center together with the circuit breakers for other circuits in the installation. However, the fault condition can occur anywhere downstream and can be further distanced from the circuit breaker and detectors by an extension cord. The wiring, and particularly the extension cord, can insert considerable resistance between the fault and the detector, which attenuates the signal sensed by the detector. When the effects of this resistance are combined with the low amplitude of the currents generated by these faults, the detectors may not have sufficient sensitivity to detect remote faults. Another problem can arise when a receptacle is not connected as intended.
Detection of an arc fault is complicated by the fact that some normal loads can produce waveforms similar to arc faults. Arc fault detectors attempt to distinguish over such phenomena to minimize nuisance faults. The task is further complicated by the fact that, as mentioned above, arc faults tend to be smaller in amplitude than dead faults. Furthermore, arc faults tend to be relatively intermittent.
With the introduction of arc fault circuit interrupter (AFCI) devices, such as arc fault circuit breakers, there exists the need for a method and apparatus for reliably determining if one of three types of circuit interrupters (i.e., AFCI, GFCI for people protection, and GFCI for equipment protection) is attached to the power circuit.
This is complicated by the fact that, unlike GFCI devices, different AFCI devices from different manufacturers have different responses to arc fault waveforms. For example, test arc fault waveforms of a particular pulse width and a particular current magnitude may require a different number of cycles or pulses in which to trip such different AFCI devices. In other words, test arc fault waveforms of a particular pulse width, a particular current magnitude and a particular count of pulses may trip some, but not all, of those different AFCI devices.
A known AFCI tester applies twelve 400 μS half-cycle pulses at 112A peak current.
There is room for improvement in AFCI tester apparatus and methods.